1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus, and an optical information recording apparatus, using a hologram capable of recording information with a higher density and a larger capacity by recording optical interference fringes on a recording medium.
2. Description of the Related Art
These days, the world has entered an age of multimedia, and the necessity for a recording apparatus that performs recording on a recording medium and a recording and reproducing apparatus that performs recording and reproducing onto and from a recording medium has increased in the level of importance, and the recording density has been raised year by year. In an optical information recording medium, also, progress has been made from a CD (compact disk) through a DVD (Digital versatile disk) to a BlueRay (Blue ray) disk. Responding to the progress made in the optical information recording media, in the optical information recording apparatus, and the optical information recording and reproducing apparatus, also using the disk, the recording density has been raised by shortening the wavelength of the light to be used. In recent years, a new recording system referred to as a holographic memory has been proposed. The holographic memory performs the recording of information by forming a hologram corresponding to the information to be recorded inside the recording medium. Because of the characteristics using the hologram, a multiplex recording is made possible, and, even if adjacent holograms have an overlapped portion with each other, information can be reproduced independently from these holograms. Consequently, the holographic memory can achieve a higher recording density, which has not been obtainable in the conventional optical information recording medium.
On the holographic memory, for example, a description is made in Hideyoshi HORIGOME, et al., [Holographic medium near at taking off the ground, 200 Gbyte expected to be realized in 2006], Nikkei Electronics, Vol. No. 891, pages 105 to 114, Jan. 17, 2005. In this article, the recording and reproducing apparatus by holographic memory using a two-dimensional spatial light modulator and a light sensing device is disclosed. Also, in this article, a description is made of an optical system of the holographic memory system (optical information recording and reproducing apparatus) of a coaxial type, referred to as a collinear system.
In the final analysis, in this optical information recording and reproducing apparatus, the information to be recorded is developed into second-dimensional digital pattern information, and by this second-dimensional optical pattern information, an information beam is modulated. By this processing, the information beam is generated, in which the recording information becomes two-dimensional spatial beam intensity distribution image information. By allowing the information beam and the reference beam to interfere, the interference fringes are recorded on the recording medium. At the reproducing time, two-dimensional digital pattern information is extracted and decoded from the beam intensity distribution image information reproduced by irradiating the reference beam. By this digital processing, it is possible to inhibit the lowering of the reproducing error rate due to deterioration of an S/N ratio, and moreover, by coding binarized data and performing error correction, it is possible to reproduce the recording information highly faithfully.
In the holographic memory system of the above-described collinear system, since the information beam and the reference beam have a coaxial optical arrangement having no angle, recording and reproducing can be performed by using one piece of an object lens. Hence, as compared to a two-axis two light beam interference system allowing the information beam and the reference beam to irradiate the recording medium from a different optical path, there is the advantage that the optical system becomes simple. Further, because of the configuration of the recording medium having a reflection film, there is the advantage that the optical system can be arranged at one surface side of the disk-shaped recording medium.